The present invention relates to an electroless Ni or Ni alloy plating solution and a method for using it. In particular, the present invention relates to an electroless plating solution suitable for forming a film having a high surface hardness on a substrate to be plated, without any heat treatment, and a plating method wherein this plating solution is used.
Known methods for plating to form a hard surface include Ni-B alloy plating method, composite plating method with boron carbide and fine diamond particles and electroless Ni-P alloy plating method. In particular, a method wherein the electroless Ni-P alloy plating is heat-treated is usually employed. However, this method has a problem that when an aluminum alloy having a low heat resistance is to be plated, the heat treatment thereof is impossible. On the contrary, the electroless Ni-B alloy plating attracts public attention, since a high surface hardness can be obtained without the heat treatment. However, this method also has a defect that the bath has a low stability.
For example, for the electroless Ni-B alloy plating, a method wherein sodium borohydride or dimethylamine borane is used is known. According to an experiment made by the inventors of the present invention wherein the plating was conducted by stirring the solution, by rocking the substrate to be plated or by barreling method, it was found that such a solution had a low stability, that Ni-B was abnormally deposited on or in the jig, barrel and plating tank and that cracks and pits were formed in the film. In addition, the continuous filtration was substantially impossible, since the abnormal deposition was accelerated. Although various methods were proposed for improving the stability of the plating solution and preventing the crack formation in the film, no method is yet practically satisfactory.
For preventing the crack formation in the film, for example, a method wherein a compound containing sulfur, nitrogen and carbon in the molecule such as L-cystine or mercaptothiazoline is added to the plating solution was proposed [Japanese Patent Unexamined Published Application (hereinafter referred to as "J.P. KOKAI") No. Hei 1-222064]. However, the effective concentration range of such a compound is quite narrow and as the concentration of the compound added becomes high, the plating is stopped unfavorably. Although it is well known that the pitting can be inhibited by adding a wettable surfactant, this effect is scarcely obtained when the plating is conducted by stirring the plating solution, by rocking of the substrate to be plated or by barrel processing.